By turns history, biography, and philosophical inquiry, this compact volume follows one of the seminal arguments of Western religious belief across centuries. In 1078, Anselm of Bec produced what we know now as the "ontological argument" for God's existence, employing the faculties of his mind rather than appealing to the Bible or church authority to state that the "idea" and "reality" of God were the same. In the 14th century, William of Ockham's notorious "razor" logically denied all such proofs not based on observable truth. Larry Witham tells the story of this intellectual quest across the Middle Ages and how it inspired the West's first modern philosopher, René Descartes.